Refining the Climate Role of Tropical Cyclones: Key Constituents of the Summer Hadley Cell?

An important focus of ongoing research in tropical meteorology is why there are, on average, 60 tropical cyclones (TCs) in the Northern Hemisphere (NH) per year and how this number may vary in response to climate change. Greater understanding of current and future trends in TC activity may be achieved by determining whether TCs have a substantial impact upon the climate. While the precise atmospheric role of TCs in climate remains uncertain, recent research has suggested that TCs may play a role in atmospheric meridional heat transports given the strong correlation between aggregate TC activity and meridional heat transports during the following winter. Building upon prior work, the present study seeks to advance our understanding of the potential climate role of TCs by quantifying whether TCs are responsible for transporting significant quantities of total energy (i.e., sum of kinetic energy, latent energy, potential energy, and sensible heat) from the NH tropics into the Southern Hemisphere tropics during the peak of TC season.

The current study utilizes storm-relative composites of vertically integrated meridional total energy transports computed from the NCEP Climate Forecast System Reanalysis for western North Pacific (WPAC) TCs to quantify the contribution of these TCs to cross-equatorial total energy transports. The composite analysis reveals that the upper-tropospheric outflow jet of the TC is responsible for significant cross-hemispheric total energy transports. Zonal integration of the composited meridional total energy transports over a distance equivalent in width to the WPAC reveals a doubling of southward total energy transports at the equator relative to climatology. The southward upper-tropospheric total energy transports by the TC are primarily comprised of dry static energy transports (i.e., sum of potential energy and sensible heat) and are much stronger than the northward lower-and-middle tropospheric transports of moist static energy (i.e., sum of latent energy, potential energy, and sensible heat) by the TC. These results suggest that WPAC TCs may serve to locally accelerate the southern branch of the Hadley cell. These results may also suggest that TCs play an important climatological role in transporting total energy out of the WPAC during late boreal summer given the presence of at least one TC, on average, in the WPAC at any given time during the middle of August to the middle of September.